Use of an Infrared Spectroscopic Method for Isotopic Analysis …
189
Samples
All uranium hexafluoride (UF 6 ) samples were provided by Oak Ridge National
Laboratory (ORNL) and were measured at 50 Torr ± 10. The known pedigrees
and isotopic content confirmatory tests were performed using mass spectrometry.
All UF 6 measurements were performed at the ORNL UF 6 test loop specifically
designed for compatibility and the safety protocols relevant to handling UF 6 gas.
Ultra-high-purity nitrogen from Air Liquide was used for purging the sample cell.
DATA Analysis
The high performance infrared system is a transmission infrared absorption instrument that uses a tunable quantum cascade laser as the light source. As with traditional
transmission infrared absorption spectroscopies, the fundamental physics governing
the absorption is defined by the Beer–Lambert law:
A(ν) = − log[T (ν)] = e(ν) b c
(7)
where A is absorption, T is transmission, ε is molar absorptivity (L/mol cm), b is
pathlength (cm), and c is concentration (mol/L) [12].
To generate a transmission spectrum, the quantum cascade laser frequency is
scanned over a 25 cm
−1 range at a set rate of 2.5 cm
−1 /sec. Light transmitted through
the sample and reference cells is simultaneously measured at 10 kHz frequency,
resulting in two intensity spectra each with 100,000 channels. Because the actual
quantum cascade laser frequency is only reproducible to within 0.01 cm
−1 , a channelto-frequency calibration must be derived for each quantum cascade laser scan. The
frequency calibration is performed by assigning the known frequencies of nitrous
oxide lines to corresponding channel positions of absorbance peaks measured in the
reference cell of the system. A list of the nitrous oxide rovibrational lines used for
calibration in the 1157 cm
−1 range (ν 2 + ν 3 ) [scan range 1145–1170 cm
−1 ] and the
1291 cm
−1 range (ν 1 + ν 3 ) [1280–1305 cm
−1 ] can be found in Esplin and co-workers
[13].
Absorbance line channel locations in the reference spectrum are identified by
examining the slope and curvature of the spectral signal. Quadratic spline fit coefficients are calculated for each channel number over a specified range of channels
and sharp absorbance signals are identified through examination of the first- and
second-order coefficients, which are related to the slope and curvature of the spectral signal. The range of channels is typically chosen to match the half-width of the
nitrous oxide lines (~7 channels). In effect, the spline fit is a transformation from
discrete data measurements, A(n), to a piece-wise continuous data representation,
A(n + η):
189
Samples
All uranium hexafluoride (UF 6 ) samples were provided by Oak Ridge National
Laboratory (ORNL) and were measured at 50 Torr ± 10. The known pedigrees
and isotopic content confirmatory tests were performed using mass spectrometry.
All UF 6 measurements were performed at the ORNL UF 6 test loop specifically
designed for compatibility and the safety protocols relevant to handling UF 6 gas.
Ultra-high-purity nitrogen from Air Liquide was used for purging the sample cell.
DATA Analysis
The high performance infrared system is a transmission infrared absorption instrument that uses a tunable quantum cascade laser as the light source. As with traditional
transmission infrared absorption spectroscopies, the fundamental physics governing
the absorption is defined by the Beer–Lambert law:
A(ν) = − log[T (ν)] = e(ν) b c
(7)
where A is absorption, T is transmission, ε is molar absorptivity (L/mol cm), b is
pathlength (cm), and c is concentration (mol/L) [12].
To generate a transmission spectrum, the quantum cascade laser frequency is
scanned over a 25 cm
−1 range at a set rate of 2.5 cm
−1 /sec. Light transmitted through
the sample and reference cells is simultaneously measured at 10 kHz frequency,
resulting in two intensity spectra each with 100,000 channels. Because the actual
quantum cascade laser frequency is only reproducible to within 0.01 cm
−1 , a channelto-frequency calibration must be derived for each quantum cascade laser scan. The
frequency calibration is performed by assigning the known frequencies of nitrous
oxide lines to corresponding channel positions of absorbance peaks measured in the
reference cell of the system. A list of the nitrous oxide rovibrational lines used for
calibration in the 1157 cm
−1 range (ν 2 + ν 3 ) [scan range 1145–1170 cm
−1 ] and the
1291 cm
−1 range (ν 1 + ν 3 ) [1280–1305 cm
−1 ] can be found in Esplin and co-workers
[13].
Absorbance line channel locations in the reference spectrum are identified by
examining the slope and curvature of the spectral signal. Quadratic spline fit coefficients are calculated for each channel number over a specified range of channels
and sharp absorbance signals are identified through examination of the first- and
second-order coefficients, which are related to the slope and curvature of the spectral signal. The range of channels is typically chosen to match the half-width of the
nitrous oxide lines (~7 channels). In effect, the spline fit is a transformation from
discrete data measurements, A(n), to a piece-wise continuous data representation,
A(n + η):